The present invention relates to a DC power device adapted for receiving AC input and generating DC output. More specifically, the present invention relates to a switching type electric power device wherein an input from a rectifying circuit, which is adapted for rectifying AC input, is subjected by a switching element to ON-OFF control and then to a voltage increase or decrease by a transforming device, and thereafter converted into DC power by another rectifying circuit. The present invention further relates to a power factor correction control device which is adapted for use in a power device for correcting the power factor.
A switching type electric power device of the type described above includes an input rectifying circuit having diodes connected to form a bridge circuit, the output of the rectifying circuit is connected to an input capacitor. The output of the input rectifying circuit is connected to a primary winding of an output converter having a transforming device which is adapted for increasing or decreasing the output voltage of the input rectifying circuit. A switching element is provided for subjecting the output of the input rectifying circuit, which is connected with the primary winding of the converter, to an ON-OFF control. The transforming device of the output converter has a secondary winding which is connected through an output rectifying circuit to a load.
In a circuit provided with such an input capacitor, input current flows only during a period wherein the input capacitor is being charged, and the current is interrupted during a period wherein the input voltage is below the voltage of the input capacitor. Therefore, even when the AC input voltage is of a sinusoidal shape, the current wave form assumes a steep shape resulting in a decrease in power factor and producing harmonic components.
It has been realized and recognized as a problem that when a commercial electric power source is utilized as the AC input power source the harmonic components produced in the switching power device are allowed to flow into the commercial power source lines to thereby distort the commercial power source. Thus, in recent years, a problem in the field of the switching power device has been to take into account this tendency and to suppress the generation of such harmonic components.
In order to solve the problem, there has been proposed to provide a power device which utilizes a rectifying circuit of a type called DITHER system wherein an AC input is subjected by means of a switching element to ON-OFF control. A typical example of a switching type power device utilizing a DITHER type rectifying circuit is the one which is described as the voltage raising circuit example 1 described in the article "Application of a DITHER Rectifying Circuit to a Switching Type Power Device" by Isao TAKAHASHI in the text for a seminar entitled "Power factor correction Circuit for Switching Type Power Device" held by the Japan Industrial Technology Center on Nov. 26 and 27, 1992.
In the known circuit, the input rectifying circuit adapted to receive AC power as an input has one of output connected through an inductor and a diode with one end of a primary winding of the transforming device, the other end of the primary winding being connected through a switching element with the other of the outputs of the input rectifying circuit. A capacitor is connected in parallel with the primary winding of the transforming device and the switching element. A point between the inductor and the diode is connected through a second diode with a point between the primary winding and the switching element. The switching element is actuated by means of a control circuit to conduct ON-OFF operation with a frequency faster than the frequency of the AC input whereby current proportional to the input voltage is allowed to flow into the capacitor during the ON period of the switching element resulting in an improvement of power factor and suppression of harmonic components.
The Japanese Laid-Open Patent Nos. Hei 3-207268 and Hei 4-127875 also disclose switching type power devices which utilize a similar DITHER type rectifying circuit.
It has been recognized that the switching type power device utilizing the aforementioned DITHER type rectifying circuit is advantageous in that a high power factor can be obtained and harmonic components can be suppressed with a circuit of a relatively simple structure. It should however be noted that the circuit of this type has a disadvantage in that the capacitor voltage changes to a significant degree depending on variations in the input voltage and the load. It has therefore been required in order to accommodate for wide ranges of variations in the input voltage and the load to vary the frequency of the control circuit for providing the ON-OFF control of the switching element in accordance with the changes in the input voltage and the load so that the variations in the capacitor voltage can be minimized. It should however be noted that making the frequency of the control circuit variable as described will make it necessary to have the transforming device, the inductor and the input and output filters of larger capacities which lead to a difficulty in control.